1. Field of the Invention
The present invention relates to data communications. More particularly, the present invention relates to a method and apparatus for loop breaking on a serial bus.
2. Background
Computer systems and other digital electronic systems often use a common interconnect to share information between components of the system. The interconnect used in such systems is typically a serial bus. The correct operation of any bus requires that there be exactly one path between any two components on the bus.
The IEEE1394-1995 standard defines one type of serial bus. IEEE Standards document 1394-1995, entitled IEEE Standard for a High Performance Serial Bus (hereinafter xe2x80x9cIEEE1394-1995 standardxe2x80x9d). A typical serial bus having the IEEE1394-1995 standard architecture includes many nodes that are interconnected by links such as cables that connect a single node of the serial bus to another node of the serial bus. Nodes interface to links via one or more ports. Data packets are propagated throughout the serial bus using a number of transactions. These transactions involve one node receiving a packet from another node via one link and retransmitting the received packet to other nodes via other links. A tree network configuration and the associated packet handling protocol ensures that each node receives every packet once.
The IEEE1394-1995 standard provides for an arbitrary acyclic bus topology. Correct operation of the bus relies on the superimposition of a hierarchical relationship which is constrained by the manner in which the nodes are connected to one another. In IEEE1394-1995, this relationship is determined during the bus configuration process.
An IEEE1394-1995 serial bus is configured in three phases: bus initialization, tree identification (tree-ID) and self identification (self-ID). During bus initialization, the general topology information of the serial bus is identified according to a tree metaphor. For example, each node is identified as being either a xe2x80x9cbranchxe2x80x9d having more than one connected ports or a xe2x80x9cleafxe2x80x9d having only one connected port. A node recognizes its status as a leaf node or a branch node immediately upon entering tree-ID. During tree identification, hierarchical relationships are established between the nodes. For example, one node is designated a xe2x80x9crootxe2x80x9d node, and the hierarchy of the remaining nodes is established with respect to the relative nearness of a node to the root node. Given two nodes that are connected to one another, the node connected closer to the root is the xe2x80x9cparentxe2x80x9d node, and the node connected farther from the root is the xe2x80x9cchildxe2x80x9d. Nodes connected to the root are children of the root. This process of identifying hierarchical relationships continues until the number of children of a node is greater than or equal to one less than the number of connected ports on the node. During self-ID, each node is assigned a bus address and a topology map may be built for the serial bus.
Typically, serial buses such as the IEEE1394-1995 serial bus require knowing what is being plugged in where. For example, the back of many electronic devices has ports for connecting the electronic devices to other electronic devices. Under the IEEE1394-1995 standard, correct bus operation is not possible if electronic devices are connected in a loop configuration. A specific function of the configuration process is therefore to determine whether a loop is present, and if a loop is found to prevent completion of the configuration process, thereby rendering the bus inoperable. The bus is rendered inoperable when a timeout occurs in the configuration process. The devices must be physically reconnected in some other way to remove the loop. The configuration process provides no indication of how to reconnect the bus in order to remove the loop. Consequently, correctly reconnecting the devices requires a detailed understanding of how the devices may properly be connected. Typical users of these devices do not have such an understanding. Thus, devices on a bus may be reconnected many times using a sort of xe2x80x9chit-or-missxe2x80x9d approach before the loop is removed.
Buses such as the IEEE1394-1995 serial bus are being used increasingly to connect products for home use. These products include televisions, stereos and other home entertainment devices. Requiring typical users of such products to know what should be plugged in where is unnecessarily burdensome. Accordingly, a need exists in the prior art for a method and apparatus for loop detection and loop breaking such that devices on a bus remain connected to their neighbors, even in the presence of one or more loops.
A method for loop breaking includes selecting a first port of a node, transmitting a first packet containing a first identifier from the first port of the node, listening for a second packet containing a second identifier for a period of time on a second port of the node, joining the first port and the node if the second identifier meets a first criteria with respect to the first identifier and quarantining the first port when the second identifier meets a second criteria with respect to the first identifier. In another aspect, a serial bus includes a plurality of nodes and a plurality of communications links interconnecting the nodes, each communications link being coupled between two nodes in a manner such that each node is coupled to every other node via one or more communications links, wherein at least one of the plurality of nodes includes at least one port coupled to a communications link and operative to transmit and receive data via the communications link, wherein a loop passing through a node is broken by internally isolating at least one port from the node, such that all ports of the node remain connected to their neighbors.